One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is the creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some electric vehicles and hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for electric vehicles (EV) and hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High power and capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). To this end, a plurality of small-sized secondary batteries (unit cells) is connected in series to each other so as to form a battery module and a battery pack. According to circumstances, the small-sized secondary batteries (unit cells) are connected in series and in parallel to each other so as to form a battery module and a battery pack.
Generally, such a battery pack has a structure to protect battery modules, each of which has secondary batteries mounted therein. The structure of the battery module may be varied based on the kind of vehicles or installation position of the battery pack in the vehicles. One of the structures to effectively fix high-capacity battery modules is based on supporting bars and end plates. This structure is advantageous in that movement of the battery modules is minimized even when load is applied toward the supporting bars. To this end, however, it is necessary to sufficiently secure rigidity of the supporting bars and end plates.
In connection with this case, FIG. 1 is a perspective view illustratively showing a conventional battery pack including a single battery module.
Referring to FIG. 1, a battery pack 100 includes unit modules 10, each of which has secondary batteries mounted therein, a base plate 20, a pair of end plates 30, and supporting bars 40.
The unit modules 10 are stacked at the top of the base plate 20 in a state in which the unit modules 10 are erected vertically. The end plates 30 are disposed in tight contact with the outer sides of the outermost unit modules 10 in a state in which the lower end of each of the end plates 30 is fixed to the base plate 20.
The supporting bars 40 are connected between the upper parts of the end plates 30 so as to interconnect and support the end plates 30.
However, the battery pack with the above-stated construction uses only a single battery module with the result that the capacity of the battery pack is low. For this reason, it is difficult for the battery pack with the above-stated construction to be applied to an external device, such as a vehicle, which needs a high power and capacity battery pack.
Meanwhile, battery packs for hybrid electric vehicles are configured in various forms based on the kind of vehicles or installation position of the battery pack in the vehicles so as to stably protect a battery cell array. Among such battery packs is a battery pack having a bucket structure, which is installed in the lower part of a trunk of a vehicle or in a depressed space defined between the lower end of a rear seat and the trunk of the vehicle.
In this case, the battery pack is located below the place at which the battery pack is fastened to the chassis of the vehicle. Consequently, it is necessary to provide a structure in which the battery pack is supported by main members and a base plate, and end plates and supporting bars are located at the front and rear of the battery pack so as to prevent the battery pack from being deformed in the front and rear direction. In this structure, the main members are bent in the shape of a bucket, and therefore, the overall structural stability of the battery pack is decided depending upon rigidity of the main members.
The rigidity of the main members may be improved by sufficiently increasing the depth of a flange or the thickness of each of the main members. However, it is not possible to sufficiently increase the depth of the flange due to the limited installation space in the vehicle. Also, increasing the thickness of each of the main members increases load of the battery pack. As a result, it is not possible to improve the structural stability of the battery pack.
Therefore, there is a high necessity for a battery pack configured to have a structure in which battery modules are located below the place at which the battery pack is fastened to the chassis of a vehicle, main members to support load of the battery pack has a bucket form, and the battery modules are arranged on a base plate in two rows, wherein structural stability of the battery pack is improved while the depth of a flange and the thickness of each of the main members are maintained.